Limiting contact between gamma rays and healthy tissue is important because?

Gamma ray is high-frequency electromagnetic radiation. It is known that gamma rays are able to damage DNA molecules in the cells by interrupting DNA strands. That characteristic of gamma rays is used in the treatment of tumors. An idea was that gamma rays will damage DNA molecules of tumor cells and, consequently, prevent uncontrollably divisions of tumor cells. But, in treatments, it is very important to limit contact between gamma rays and healthy tissue. This way it will be minimized damage of DNA molecules in healthy cells.